Antigens are molecules that an immune system can recognize and target for destruction if necessary. A B antigen refers to a molecule that can be used to separate red blood cell types into groups. There are four blood groups — A, B, AB and O — and every human has one of these types. Different blood groups can interact badly with each other, so it is necessary to find out which blood type a person is before administering a transfusion.

Blood groups are genetically inherited, which means a baby receives one blood group gene from his or her mother and one from the father. A gene for B antigen cells from one parent can be inherited along with another B antigen gene from the other. In this case, the baby will be blood group B. If the baby inherits one B antigen gene and one O antigen gene, he or she will also be blood group B. Parents who give their child one B antigen and one A antigen will have a kid who is blood group AB.

Humans usually have a matching pair of 23 chromosomes in their cells. The blood type genes are found on chromosome 9. Minor differences in the sequence of the gene result in different versions of molecules called glycotransferases, and this results in the presence, or absence, of antigens on the outsides of the cells.

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Red blood cells of a type B child have B antigen molecules protruding from the cell. Blood group A people have A antigens, and AB people have A and B antigens. Type O people have none of these antigens.

Circulating in a type B person's blood are antibodies to the A antigen. These antibodies target A antigens and stick to them. This causes the A blood cells to clump up and can result in dangerous medical issues.

Type AB people have none of these antibodies. Therefore, a person with blood type B can receive blood from someone with type B or type O blood but not from AB or A people. Antibodies to other blood types arise even before someone gets a blood transfusion because similar antigens are produced by bacteria and possibly by certain plants.

The reason for the range of blood types may lie in the fact that people with certain blood groups are more resistant to specific diseases than those with other blood types. A beneficial mutation could have given one person an advantage in surviving disease over evolutionary history. Blood type O people are more at risk from cholera and plague than the other blood groups. Type A blood is more susceptible to smallpox and some cancers than people with O or B antigens.

Apes also share some of the same blood types as humans. Baboons have A, B, and O blood types, but chimpanzees usually have A, rarely have O, and never have B. Gorillas, on the other hand, only have type B blood.

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